Single side band and carrier transmitter



Feb.r3, 1942. D, POLLA'CK 2,272,068

SINGLE SIDUBAND AND CARRIER TRANSMITTER Filed June 279," 1959 num. Y

l' 'I Y :inventor Daze Vlfazacfi attorney Patented Feb. 3, 1942 TRANSMITTER Dale Pollack, Haddon Heights, I'N..J., assignor to Radio Corporation of America, a corporation ,k

of Delaware Application June 29, 1939, Serial No. 281,919 v 1 4 claims. `(c1. 250517)' This invention relates to single side-band and carrier transmitters and more particularly to a method of and a means for producing efficiently a single side-band and radio frequency carrier.

There .are a number of systems or methods for producing single side-band and carrier signals. One method is to filter `the output of a conventional amplitude modulated signal to thereby eliminate one side-band. Another method is to lter an amplitude-modulated Wave in the low power stages of a transmitter and to amplify the filtered signal, which may be subsequently radiated or used to modulate a second radio frequency oscillator from which the desired carrier and side-band currents are derived. The former method is not very eicient andmay either leave an undesired vestigial side-band or may attenuate the carrier. The latter'system does not make eiiicient use of the amplifiers which include currents of undesired as well as desired frequencies.-

Among the objects of theinvention is the provision of means for obtaining efficiently asingle side-band and radio frequency carrier.- Another object is to provide means for amplifying'separately and combining currents of carrier fre-f quency and currents of one side-band. An additional object is-to provide means for filtering currents of one 'side-bandfrequency from an amplitude-modulated carrier and combining said currents with currents of carrier frequency -in suitable phase to thereby establish a signal of single side-band and carrier frequences.- A

The invention Will bedescribedby reference to the accompanying drawing in which Figure y1 is a schematic diagram of one embodiment of the invention; Figure 2 is a schematic diagram ofthe invention showing the system for creating the carrier and side-bandcurrents; Figure 3 is a circuit diagram of Ybalanced modulator used in the invention; Figure k4 is aschematic `repre,- sentation of an impedance inverter; and Figure 5 is a circuit diagram of a high pass filter.

Referring to`Fig.-1, aj-'source I of carrier fre--` quency voltage is appliedfto the input of an'amplier A Whose output ris connected through an impedance inverter 3 to aload circuit 5.' The load circuit is also connected to the output of a second amplifierB. The input of the second amplifier B is connected toa source 1 of single sideband voltage. The i'lrst amplifier is preferably operated as a class-C device to thereby obtain high eiiciency. The second amplifier B is biased to cut-off so that it is inoperative in the absence of 'a modulation or side-band signal.

In the operation ofthe device, the first amllselectsthe upper side-bandof 3 to5.5 megacycles and rejects the lower side-band of .5to 3.

plifier A applies currents of carrier frequency to the loadV 5 through the impedance inverter 3.

Since the secondamplier B is biased to cut-olf during the absencefo'f signal, it will have no effect on .the carriersource or load circuit. When the side-band signalsl are applied to the second amplier, amplied radio-frequencysignals of sideband frequency will appear, in its output circuit and hence in the load circuit.v At some instants, theside-band voltages Will be -in phase with the carrier voltages, and thereupon the carrier and side-band currents in the load circuitwill combine and the'load circuit'outputfwill increase. At other instants,the.phases of the carrierand side-band voltages will be opposed and therefore the load circuit output will be reduced. The impedance'. inverter prevents rundesirable reactions of the amplifiers upon each other and, furthermore, reduces `the output from the `ampliiier A ras the loadfoutput is reduced. At the trough of the cycle, the `rst amplii'lerjA contributes no power and f thus carrier kand side-band currents are'obtained efficiently and Without substantial distortion. v`

` While the foregoing description applies to pure Y `frequency comparedt'o'the final carrier. By Way rof example, the transmitter is assumed to befa vtelevision station operating onl a carrier of 63l megacycles and have a single side-band modulation range of 63 Ato'6v5.5 megacycles. vBased upon this assumption, the master oscillator frequencyl is 3 megacycles per'second`.` `The master oscilla.- tor Il and a modulation source I3 are connected to va balanced modulator I5. A filter I1 connected to the output of the balanced modulator megacycles. This rupper side-band is applied to a; second balanced modulator I9 which is connected to a secondoscillator- '2 I` off'a frequency of 10 megacycles persecondr4 The' output of the second balanced modulator is connected -toa lter 23 which passes the upper side-band (I3-15.5 megacycles) and rejects the lower side-band ('I-lil megacycles) k.

The last-mentioned lter 23 and a third oscil lator 25, operating` at afrequency of 50 megacycles per second, are connected to athird balanced modulator 2l.

The output of the third balanced modulator. is connected to a third lter 3| which passes the upper side-band (6B-65.5 megacycles) and rejects the lower side-band (34.5-3'7 megacycles). The output of the third filter 3| is applied to an amplifier 33 which is biased to cut-olf when no modulation is being applied. The output of the amplifier 33 is applied to an antenna 35 or other load circuit.

The master oscillator is also connected, through a frequency multiplier 3l, to the main carrier frequency generator 39. The output of the carrier frequency generator 39 is applied, through a phase shifter 4|, to the input of an amplifier 43. The amplifier 43 is biased preferably for class C operation. The output of the amplifier is connected, through an impedance inverter 45, to the antenna 35 or other load circuit.

In the foregoing system, the phasing network 4|, connected between the carrier generator and the impedance inverter, is used to insure proper phase relations if any vestigial carrier is present. It will be seen that the carrier frequency is synchronized with the master source so that proper phasing exists between the carrier and its accompanying single side-band. The balanced modulator, shown in Fig. 3, is well known to those skilled in the art. A suitable impedance inverter may consist of the quarter wave transmission line of Fig. 4 or the equivalent network in concentrated reactances. A high pass filter may consist of two or more mutually coupled resonant circuits of the type illustrated in Fig. 5, or the equivalent thereof. It should be understood that different numbers of oscillator-balanced modulator and filter elements may be used.

It should be understood that the invention is not limited to the precise means illustrated for obtaining the single side-'band currents. For example, the method disclosed in the copending application Serial No. 248,567, filed December 30, 1938, by D. G. C. Luck, for Antenna circuit modulation systems, may be used in place of the single side-band source shown in Fig. 2 or both methods may be combined. In some installations, the use of dual balanced modulators is preferred at the lower modulating frequencies and the cascade filter type of selection, shown in Fig. 2, is preferred at the higher modulation frequencies; i. e., both methods are used in combination. In any event, the synchronized carrier and side-band currents are combined in accordance with the instant invention.

I claim as my invention:

l. A single side-band and carrier transmitter including in combination a source of carrier frequency currents, a source of modulation currents, means for combining said currents, means for filtering currents of a single side-band from said combined currents, a second carrier source, means for applying said single side-band currents to said second carrier source to obtain a second carrier current modulated to include a 'pair of side-bands, means for selecting currents corresponding to one of said pair of side-bands, a single side-band amplifier having its input biased to cut-off and its output connected to a load circuit, an impedance inverter connected to said load circuit, a carrier amplifier having its input connected to said carrier source and its output connected to said impedance inverter, means for applying said selected side-band currents to said single side-band amplier, and means for adjusting the phase of the first carrier currents with respect to the selected single side-band currents so that a desired phase relation exists between the carrier currents and side-band currents in said load circuit.

2. A transmitter of the character of claim 1 including means for biasing said carrier amplifier to thereby obtain class C amplifier operation.

3. A single side-band and carrier transmitter including in combination a source of oscillatory currents of carrier frequency, a source of modulation frequency currents, a balanced modulator, means for applying said oscillatory currents and said modulation currents to said modulator s0 that upper and lower side-bands are obtained, a filter for passing one of said side-bands and rejecting the other, a second carrier source, means for applying said single side-band currents to said second carrier source to obtain a carrier current and currents corresponding to a pair of sidebands, means for selecting the currents of one of said pair of side-bands, a first amplier, a second amplifier, a load circuit connected to the output of said second amplifier, an impedance inverter connected to the output of said first amplifier and said load circuit, means for applying said carrier currents to the input of said first amplifier, means for applying the selected one of said side-band currents to the input of said second amplifier, and phasing means for adjusting the relative phases of the currents applied to said first and second amplifiers so that desired phasal relations between the carrier currents and selected one of said single side-band currents is maintained.

4. A single side-band and carrier transmitter including in combination a lirst amplifier, a second amplifier, a load circuit effectively connected to the outputs of said amplifiers, an impedance inverter connected between the output of one of said amplifiers and said load circuit, a source of carrier frequency current, a source of modulation currents, means for applying said modulation currents to said carrier source to obtain a modulated carrier including a pair of side-bands, means for selecting currents of one side-band from said modulated carrier, a second carrier, means for applying said selected side-band currents to said second carrier to obtain a modulated carrier including a second pair of side-bands, means for selecting currents of one of said second pair of side-bands, and means for impressing currents derived from the first carrier source on the input of one of said amplifiers and the last selected side-band currents on the other of said amplifiers, means for adjusting the phase of said derived carrier currents with respect to said last selected single side-band currents so that the desired phase relations are maintained between the output currents of said rst and second ampliflers.

DALE POLLACK. 

